Olefins are often formed by catalytic dehydrogenation reactions, for example, the styrene monomer is generally formed by the reaction of ethylene and benzene in the presence of aluminum chloride to yield ethylbenzene, which is then catalytically dehydrogenated at about 600.degree. to 700.degree. C. to form styrene. Alternatively, styrene may be formed by the chlorination of ethylbenzene and the subsequent removal of hydrogen chloride. However, these methods require substantial care and expense with regard to the elimination of byproducts such as phenylacetylene and alpha-methylstyrene, each of which are particularly troublesome when present during the formation of the various styrene polymers.
Moreover, the demand for ethylbenzene is sufficient to occasion a need for a styrene-forming reaction with an alternative and less expensive starting material.
U.S. Pat. No. 2,110,830 to Dreisbach details the thermolysis of isopropyl benzene in the presence of steam at a temperature of from 750.degree. to 950.degree. C. for the simultaneous formation of styrene and phenylacetylene, and particularly teaches that the yield of phenylacetylene increases with the reaction temperature. The separation of the phenylacetylene from the styrene is then attained by additional purification steps, i.e., by distillation or precipitation.
U.S. Pat. No. 2,441,095 to Cheney et al., describes temperatures of from 650.degree. to 900.degree. C. at pressures of from about 15 to 100 psi to attain a catalytic dehydrogenation of isopropyl benzene to form styrene and alpha-methylstyrene. As noted, alpha-methylstyrene inhibits the polymerization of the various other styrenes due to the presence of the methyl group on the alpha-carbon.
Accordingly, it has been a desideratum to provide a direct process for the conversion of C.sub.8 and C.sub.9 aromatics to polymerizable styrenes, particularly styrene and para-methyl styrene, without the formation of byproducts which interfere with further use of the product. More generally, it would be advantageous to provide a similar method for the formation of styrene and polymerizable styrene derivatives from products which are less in demand than cumene and other known starting materials, e.g., from plentiful materials such as xylenes.
According to the present invention, the gas-phase thermolysis of a variety of C.sub.8 and C.sub.9 aromatics gives styrene and/or polymerizable methylstyrenes in yields of up to 98% at high conversion rates. Aromatic substrates include ortho-xylene, meta-xylene, para-xylene, mixed xylenes, benzocyclobutene, cumene (isopropyl benzene), ortho-alkyltoluene, meta-alkyltoluene, para-alkyltoluene, mixed alkyltoluenes, ortho-cymene, meta-cymene, para-cymene, mixed cymenes, and functionalized derivatives thereof. These substrates include compounds which are in lower demand than the starting materials currently in use for the production of styrenes. In particular, mixed xylenes are currently the least expensive aromatic compound available, and thus is a precursor of choice if there were an economical conversion process.
Thus, C.sub.8 and C.sub.9 precursors in a gas-phase are thermolyzed at temperatures in excess of about 900.degree. C. at low pressures, or in excess of about 800.degree. C. in the presence of inert diluents or dehydrogenating vapors, in a one-step process for the formation of polymerizable styrenes.
The terms "thermolysis" and "thermolytic", as used herein, should be understood to refer to the transformation of the aromatic compound by heat alone, i.e., without oxidation in the sense of oxygen combining with the starting compound. While such a process may also be referred to as pyrolysis, we prefer the term "thermolysis" in that it literally refers to the application of heat rather than fire. Although either of these terms imply decomposition into smaller fragments, thermolytic change may also involve isomerization and the formation of higher molecular weight compounds.